Cache data loaded from MDK data files for faster loading

[Calinou]

Since MDK's data files are loaded and their contents interpreted in GDScript, this is a slow operation done on every startup. This in turn impacts iteration times when working on the project using the Godot editor. We can make this startup cost only happen once by caching this data to user://.

Since the MDK data never changes, this cache should be active both when running from the editor and in exported builds. There should be a --no-mdk-cache command line argument to disable this cache if needed.

Ideally, the cache generation process should display a progress bar, but displaying a static "Caching MDK game data, this may take a while…" label is acceptable too.